This invention relates generally to light detectors. More specifically, this invention relates to precision, wavelength tunable light detectors that can be used in fiber optic communications and spectroscopy.
Wavelength division multiplexing (WDM) is a technology used in fiber optic communications to increase the data capacity of an optical fiber. In WDM, several distinct wavelengths are simultaneously passed through the optical fiber. Each wavelength corresponds to a single channel. In order to implement WDM, an array of lasers is required at the transmitting end of the fiber, and an array of photodetectors is required at the receiving end. Each laser and each photodetector is tuned to a certain wavelength. Semiconductor lasers are typically used as transmitters, and the linewidth of such lasers is quite narrow, typically about 0.1-1.0 Ghz, or 0.003-0.033 Angstroms. The linewidth of semiconductor lasers is not the limiting factor in the channel spacing (channels per unit wavelength). Rather, the channel spacing is limited by current receiver technologies.
Typical receivers for WDM systems have a diffraction grating that separates optical signals into distinct wavelengths. In such systems, the channel spacing typically must be at least 2 nm for adequate resolution. Also, it is important for photodetectors in WDM applications to have high sensitivity and high speed operation. Present photodetectors cannot provide high speed, high sensitivity and narrow linewidth sensitivity. In particular, photodetectors having a linewidth narrower than 2 nm would therefore be quite useful in WDM systems.
Some WDM systems have a graded photodetector array with each photodetector having a different physical structure (e.g. material composition) rendering each photodetector sensitive to a distinct wavelength range. Typically, the arrays are made by nonuniformly distributing dopants onto a substrate wafer. The concentration of the dopant influences the wavelength range of the photodetectors. It is difficult to nonuniformly and precisely affect the properties of the photodetectors on a single substrate. Therefore, such photodetectors arrays have a high cost.
Another problem with graded photodetectors arrays is that since each photodetector is unique, each photodetector requires a backup duplicate for reliability. This also tends to increase the cost of graded array WDM systems. It would be an advance in the art of WDM systems to provide photodetectors arrays that provide high reliability with a reduced number of photodetectors. In particular, it would be an advance in the art of WDM systems to provide photodetectors that can replace any other photodetector in a WDM photodetector array.
Also, it would be an advance in the art of photodetectors generally to provide photodetectors having a combination of high speed and high sensitivity. Presently, solid state photodetector designs present undesirable performance tradeoffs between speed and sensitivity. It would be particularly useful for such devices to have narrow bandwidth.
It would also be an advance in the art of photodetectors to provide tunability in a solid state photodetector. Tunable photodetectors could be used in a wide range of applications, particularly if they have high speed, high sensitivity, and narrow bandwidth.
Photodetectors with a combination of high speed, high sensitivity, tunability and narrow bandwidth could be used in a wide range of applications including WDM, spectroscopy, and free space optical interconnections.
Accordingly, it is a primary object of the present invention to provide a photodetector that:
1) has a narrow linewidth;
2) has a high quantum efficiency and high sensitivity;
3) has a high speed;
4) is tunable over a broad range of wavelengths;
5) is suitable for dense wavelength division multiplexing applications and spectroscopy applications; and
6) provides gain.
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
These objects and advantages are attained by an apparatus for detecting light having a front reflector for receiving incident light. The front reflector has a reflectivity of Rf. The apparatus also has a back reflector with reflectivity Rb. The front and back reflectors are aligned to form an optical resonator. A photodetector is disposed within the resonator, and the photodetector has a double pass absorption of A. In the present invention, a ratio Rf/Rb is equal to the value (1xe2x88x92A) to within 10%.
Preferably, Rf is greater than 75%, more preferably 90%, or most preferably 98%. Also, the ratio Rf/Rb is preferably within 5% of the value (1xe2x88x92A), more preferably within 1% of the value (1xe2x88x92A).
Preferably, the double pass absorption is less than 25%, 10%, 5%, or 2%. Smaller values are more preferred.
Preferably, the resonator is an optical microcavity. Preferably, the distance between the front and back reflectors is less than 10 microns, more preferably, less than 2 microns.
The photodetector can be a PIN photodiode, phototransistor, avalanche photodiode or any other photodiode photodiode. The photodetector may comprise a quantum well.
The photodetector can comprise a discrete absorption layer. Preferably, the absorption layer is less than 300 nm thick, more preferably, less than 100 or 50 nm thick.
Preferably, at least one of the reflectors is movable such that a distance between the front reflector and back reflector is adjustable and the device is tunable. A flexible membrane can provide tunability.
The reflectors can be distributed Bragg reflectors.